This invention relates generally to a coupling for resonant cavities and more particularly to a coupling mechanism.
An RF resonant cavity (or multiple interconnected cavities) can be used to create RF filters. The filters may either pass a RF signal over a limited frequency range (a bandpass filter) or exclude an RF signal over a limited frequency range (a notch or bandstop filter), depending upon how the resonator is connected to the overall system. A perfect single cavity device would operate at a single, specific frequency (the resonant frequency), however due to material and other considerations all resonant frequency devices operate over a frequency range which encompasses the resonant frequency.
One type of RF resonator is realized by having a conductive post within an enclosed conductive cavity. The post is connected to the housing at one end and extends towards the top of the cavity. Generally, the resonant frequency of the cavity is selected by adjusting the length of the post.
As mentioned previously, a single resonant cavity only passes energy over a very limited range of input frequencies. A practical device needs to allow energy over a broad range of frequencies to pass, while still blocking energies above and below this frequency range. This is accomplished by combining or coupling multiple cavities together. This causes the filter response curve to widen about the resonant frequency.
The cavities are combined by coupling a portion of the energy from one cavity into an adjacent cavity. Standard equations, tables or design guidelines are used which describe the amount of energy that must be coupled in order to realize a given performance goal. The energy is coupled by opening a window or iris between two adjacent cavities, so that the electromagnetic field within one cavity creates a field within the other cavity. The physical size and location of the iris is what controls the amount of energy coupled between the adjacent cavities.
Due to limitations in the ability to manufacture a perfect device, it is necessary to have a method or means for adjusting the energy coupled through the iris. Any obstruction or protrusion in the iris will affect the coupling between the cavities. The obstruction acts as a magnifier for the energy, increasing the amount of energy coupled between the cavities. By altering the relative size or location of the obstruction within the iris it is possible to adjust the coupling to satisfy the given performance requirements.
Typically the adjustment of the coupling is accomplished by using a threaded rod extending from the lid or housing into the iris. U.S. Pat. Nos. 5,841,330 and 5,805,033 show typical coupling mechanisms. The end of the rod may be either straight, as shown in the ""330 patent, or it may have a disk or other protrusion at its end, as shown in the ""033 patent. Tuning is accomplished by moving the rod into or out of the iris, until the desired performance is met. The rod is then locked in position using a locking nut. This tuning method is undesirable because it requires sufficient room be provided for the portion of the rod that extends above the surface of the filter, thus increasing the overall size of the space required to house the resonator.
A threaded rod decreases the voltage stability of the device. The coupling element is located in an area of high electric field strength. The sharp profile of the thread can not support a high electric field without breakdown.
It is an object of the present invention to provide an improved mechanism for coupling resonant cavities.
It is a further object of the present invention to provide a compact coupled cavity resonator.
It is still another object of the present invention to provide an iris coupled cavity resonator which can be operated with high electric fields at the coupling iris.
There is provided a cavity resonator including a housing having interior cavities with top and bottom walls in which adjacent cavities are coupled to one another through a coupling window having spaced sidewalls and a smooth coupling element supported from the top wall in said window for lateral adjustment with respect to the side walls.